Optical interferometers are roughly classified into optical interferometers based on amplitude-splitting and those based on wavefront-splitting. However, as far as electron interferometers are concerned, only the wavefront-splitting type interferometers, each of which uses one electron biprism, are brought into actual use. This type of interferometer cannot independently control the interference fringe spacing s and the interference width W in principle. Therefore, if the wide interference width is required, for example, in the case of a large specimen, an interferogram (hologram), whose interference fringe spacing is narrow, and which is composed of a large number of interference fringes, must be analyzed. As a result of this, an image recorded with the high carrier-spatial frequency had to be analyzed. In contrast with this, even if an interferogram with the high carrier-spatial frequency is required in a narrow space because a specimen is small, when the required high carrier-spatial frequency is reached, the interference width becomes wider, causing the spatial coherence to be degraded. Accordingly, a low quality interferogram composed of low-contrast interference fringes had to be analyzed.
With the objective of addressing these problems, the inventors of the present invention applied for the invention of PCT/JP2005/000111. FIG. 1 is a diagram illustrating an interference system that uses electron biprisms described in FIG. 3 of PCT/JP2005/000111. This will be roughly described here as the background art of the application concerned.
In FIG. 1, reference numeral 1 denotes an electron source; reference numeral 2 denotes an optical axis; reference numeral 3 denotes a specimen; reference numeral 5 denotes an objective lens; reference numeral 7 denotes an image plane of an electron source over the upper electron biprism; reference numeral 11 denotes an observation plane; reference numeral 12 denotes an image of a specimen on an observation plane; and reference numeral 13 denotes an image recording device such as film and a camera. Reference numerals 21 and 23 denote an object wave and a reference wave respectively. Reference numeral 31 denotes an image plane of a specimen determined by the objective lens 5; reference numeral 32 denotes an image of a specimen formed by the objective lens 5; reference numeral 33 denotes a magnifying lens; reference numeral 35 denotes an image plane of an electron source determined by the magnifying lens 33; and reference numeral 9u denotes a filament electrode of an upper electron biprism that is placed on the image plane 31 of a specimen determined by the objective lens 5, and a diameter of the filament electrode 9u is du. Reference numeral 9b denotes a filament electrode of a lower electron biprism that is placed between the image plane 35 of the electron source determined by the magnifying lens 33 and the observation plane 11. A diameter of the filament electrode 9b is db. In addition, the interference fringe spacing s and interference width W appearing on the observation plane 11 are schematically illustrated below the image recording device 13. Here, in the figure, although the electron source 1 is shown as a single block, the block includes a source, an acceleration tube, and a condenser optical system. The electron biprism schematically illustrated here is an electric field type. This electron biprism is constructed of a filament electrode located in the center, and right and left grounded electrodes located far outside an area through which an electron beam near the central filament electrode passes. An electron beam is deflected by applying the voltage to the central filament electrode. FIG. 1 and figures thereafter illustrate only a cross section of the central filament electrode with a small circle. In addition, in the description in which a function of an electron biprism is paid attention to, it is expressed only as an electron biprism. Further, in the description in which the central filament electrode is paid attention to, the central filament electrode is expressed as a filament electrode of an electron biprism. Moreover, because electron optical systems usually use a magnetic-field-type lens as an electron lens, said an electro-magnetic lens, a path of an electron beam includes the azimuth rotation whose rotation center is an optical axis. However, an identical plane is illustrated in FIG. 1 as an electron optical system with the azimuth rotation of an electron beam caused by the electromagnetic lens being disregarded. The figures thereafter, each illustrating an optical system, are also treated in the same manner.
An electron beam generated by the electron source 1 is divided into the object wave 21 passing through a specimen 3 located on the one side of the optical axis 2, and the reference wave 23 passing through the other side of the optical axis 2 where the specimen 3 does not exist. For easier identification of the object wave 21 and the reference wave 23, only the object wave 21 is provided with pattern display. The object wave 21 and the reference wave 23 are refracted by the objective lens 5, and then intersect each other at the image plane 7 of an electron source over an upper electron biprism to travel towards the magnifying lens 33. The object wave 21 and the reference wave 23 form the specimen image 32 on the image plane 31 of the specimen, the image plane 31 being determined by the objective lens. In addition, the object wave 21 and the reference wave 23 pass through a position of the upper electron biprism on the image plane 31. The deflection by the voltage applied to the filament electrode 9u of the upper electron biprism causes each electron beam to go towards the optical axis 2. Reference numerals 25 and 27 denote virtual electron-source images of both the object wave 21 and the reference wave 23, which are deflected by the filament electrode 9b of the lower electron biprism. Reference numerals 26 and 28 denote real electron source images of both the object wave 21 and the reference wave 23, which are deflected by the filament electrode 9u of the upper electron biprism. Yr is the split distance from an optical axis to the electron source image 26 or 28 caused by the deflection by the filament electrode 9u of the upper electron biprism. Yv is the split distance from the real electron-source image 26 to the virtual electron-source image 25 caused by the deflection by the filament electrode 9b of the lower electron biprism. Yr and Yv are expressed by equations (1), (2) as follows.
                    [Equation  1]                                                                      Y          r                =                                                            b                3                                            a                3                                      ·                                          α                u                            ⁡                              (                                                      b                    Obj                                    -                                      b                    1                                                  )                                              =                                                                      b                  3                                                  a                  3                                            ·                              α                u                                      ⁢                          D              u                                                          (        1        )            
[Equation 2]Yv=αb(bM−b3−Lb)=αb(Db−Lb)  (2)
Because an observation plane 11 is an image plane of the filament electrode 9u of the upper electron biprism, the deflection by the upper electron biprism does not influence the image formation. Accordingly, no wavefront overlap occurs. However, because the deflection to the electron beam actually works, splits 26 and 28 of the real electron-source images occur. These are essentially the same as splits 25 and 27 of the virtual electron-source images by the filament electrode 9b of the lower electron biprism.
When both of the biprisms 9u and 9b simultaneously work in the configuration shown in FIG. 1, interference fringes which are reversely projected on a specimen surface are expressed by equations (3), (4). Hereinafter, the interference fringe spacing s and interference width W which are reversely projected on the specimen surface are expressed with a subscript Obj.
                    [Equation  3]                                                                                                                s                Obj                            =                                                                                          a                      M                                                              b                      M                                                        ·                                                            a                      Obj                                                              b                      Obj                                                        ·                                                                                    D                        b                                            ⁢                      λ                                                                                      Y                        v                                            +                                              Y                        u                                                                                            ⁢                q                                                                                        =                                                                    a                    M                                                        b                    M                                                  ·                                                      a                    Obj                                                        b                    Obj                                                  ·                                                                                                    a                        3                                            ⁡                                              (                                                                              b                            M                                                    -                                                      b                            3                                                                          )                                                              ⁢                    λ                                                        2                    ⁢                                          {                                                                                                    α                            b                                                    ⁢                                                                                    a                              3                                                        ⁡                                                          (                                                                                                b                                  M                                                                -                                                                  b                                  3                                                                -                                                                  L                                  b                                                                                            )                                                                                                      +                                                                              α                            u                                                    ⁢                                                                                    b                              3                                                        ⁡                                                          (                                                                                                b                                  Obj                                                                -                                                                  b                                  1                                                                                            )                                                                                                                          }                                                                                                                                              =                                                1                                      M                    M                                                  ·                                  1                                      M                    Obj                                                  ·                                                                            a                      3                                        ⁢                                          D                      b                                        ⁢                    λ                                                        2                    ⁢                                          {                                                                                                    α                            b                                                    ⁢                                                                                    a                              3                                                        ⁡                                                          (                                                                                                D                                  b                                                                -                                                                  L                                  b                                                                                            )                                                                                                      +                                                                              α                            u                                                    ⁢                                                      b                            3                                                    ⁢                                                      D                            u                                                                                              }                                                                                                                              (        3        )            
                    [Equation  4]                                                                                                                W                Obj                            =                                                                                          a                      M                                                              b                      M                                                        ·                                                            a                      Obj                                                              b                      Obj                                                        ·                                                            2                      ⁢                                              Y                        v                                            ⁢                                              L                        b                                                                                                            D                        b                                            -                                              L                        b                                                                                            -                                                                            a                      Obj                                                              b                      Obj                                                        ⁢                                      d                    u                                                                                                                          =                                                                                                                  a                        M                                                                    b                        M                                                              ·                                                                  a                        Obj                                                                    b                        Obj                                                              ·                    2                                    ⁢                                      α                    b                                    ⁢                                      L                    b                                                  -                                                                            a                      Obj                                                              b                      Obj                                                        ⁢                                      d                    u                                                                                                                          =                                                                                          1                                              M                        M                                                              ·                                          1                                              M                        Obj                                                              ·                    2                                    ⁢                                      α                    b                                    ⁢                                      L                    b                                                  -                                                      1                                          M                      Obj                                                        ⁢                                      d                    u                                                                                                          (        4        )            
The equations (3) and (4) mean that the interference width WObj does not depend on the deflection angle αu formed by the upper electron biprism. This makes it possible to independently control the interference fringe spacing sObj and the interference width WObj. To be more specific, the independent control becomes possible by the steps of:
(1) for the lower electron biprism, determining the interference width WObj; and
(2) for the upper electron biprism, adjusting the interference fringe spacing sObj.
Moreover, when considering a case where the filament electrode 9b of the lower electron biprism is placed at a position of the image plane 35 of the electron source determined by the magnifying lens 33 (Db−Lb=0), the interference fringe spacing sObj does not depend on the deflection angle αb according to the equation (3). To be more specific, under these optical conditions, the interference fringe spacing sObj and the interference width WObj can be completely and independently controlled.